Go to the app store to install the Order Printer app.
Shopify's Order Printer app is a free and simple app that allows you to print custom invoices, labels, receipts, packing slips, and more.
The app provides templates for an invoice and a packing slip. You can include the documents in shipments when you fulfill orders or give them to your customers at a point of sale.
You can customize the templates by using HTML/CSS and Liquid code. Shopify can help you with minor customizations, but larger customizations will need to be done by an experienced third party designer.